Optical fibers are in widespread use today as transmission media because of their large bandwidth capabilities and small size. However, they are mechanically fragile, exhibiting fracture under some forms of tensile loading and degraded light transmission when bent. The degradation in transmission which results from bending is known as macro- or microbending loss. Accordingly, cable structures have been developed to protect mechanically the optical fibers thereby rendering them a reliable transmission medium in various environments.
Because of the broadband characteristics of optical fibers, together with the relatively few repeaters per length of cable which are required in a transmission system of significant length, optical fibers are well suited for intercontinental and other undersea communications. Of course, optical fibers used in such environments must be packaged in a suitable, cost effective cable structure.
Underwater or submarine cables as they are often called have special requirements. One major challenge confronting optical fibers used in submarine cables is the need for hermetic protection of the optical fibers against moisture. This is important because of the mechanical stresses experienced by an underwater cable during laying and recovery. Through a phenomenon known as static fatigue, the combination of moisture and stress on an optical fiber has been found to degrade the strength of optical fiber. Further, an underwater optical fiber cable must include provisions for an efficient direct current (D.C.) path for powering optical repeaters in the system and must be sufficiently strong to withstand the above-mentioned stress and years of operation in an underwater environment.
The foregoing problems have been overcome by a submarine cable for optical communications which is disclosed in U.S. Pat. No. 4,156,104 which issued on May 22, 1979 in the name of R. C. Mondello. In it, a core comprises a longitudinally extending strength member which may be a high strength conductive or dielectric material and which is referred to as a kingwire. A plurality of helically stranded optical fibers are embedded in a layer of a plastic material such as Hytrel.RTM. plastic material which is disposed about the kingwire. About the layer of plastic material may be disposed a protective cover of nylon or other relatively high melting point material. Several layers of steel wires are stranded about the protective cover and are held in engagement with the protective cover by a tubular copper shield and are barrier which hermetically seals the core. The copper barrier also is effective to provide a conductive path for powering repeaters and also is effective to hold together components of the cable package. The wires, especially those for an innermost layer, are effective to provide resistance to the hydrostatic pressures experienced by the cable during use. A jacket comprising polyethylene plastic material is disposed about the hermetic barrier. If desired for added toughness, a separate higher density polyethylene outer jacket may be included. In the manufacture of such a cable, a waterblocking material is interspersed among the wires and between an inner layer of the wires and the core.
The prior art also includes U.S. Pat. No. 4,484,963 which issued on Nov. 27, 1984, in the names of S. N. Anctil, R. F. Gleason, D. A. Hadfield, J. S. B. Logan, Jr., and A. G. Richardson. In it, a core comprises a kingwire which is enclosed by an elastomeric material in which are embedded a plurality of optical fibers. A protective cover of nylon applied to the elastomeric material has a layer of adhesive material applied thereto. The adhesive material forms a tight bond between the nylon cover and a first layer of steel wires which are stranded about the cover. This bond prevents creep and assures that the core tracks the steel wires during cable laying, cable recovery and in-service operations. A second layer of steel wires is stranded about the first layer and a conductive tubular member made of copper is formed thereover. A plastic jacket is disposed about the copper tubular member.
The foregoing cable structures may be classified as being tightly coupled because the optical fibers are tightly coupled to the kingwire through the plastic material and thereby experience substantially the same strain as other cable components. However, accessing individual optical fibers is somewhat difficult in such cables. Further, compressive forces which are applied laterally to the core during the manufacture of the cable are experienced by the optical fibers through the plastic material in which the fibers are embedded.
Alternate arrangements have been used in underwater cables. One of these is a cable referred to as a slotted core optical cable in which a centrally disposed core member is made of a plastic material and includes a plurality of optical fiber receiving grooves with each groove opening to an outer surface of the core member. One or more optical fibers is positioned in each of the grooves. A viscous filling material is injected into each groove and allows relative movement between the fibers and the core member to occur. See U.S. Pat. No. 4,548,664. In another slotted core cable structure, the grooves are partially filled with a material which is viscous at cable operation temperature but which is cooled to reversibly harden the material. Optical fibers are positioned in each groove after which each groove is topped out with the viscous material. See U.S. Pat. No. 4,422,889.
Slotted core cables facilitate access to the optical fibers and isolate optical fibers in the grooves from laterally applied compressive forces. However, in contrast to the cable of the Mondello patent, the slotted core cables just described are characterized by a loose coupling between the optical fibers and the core member.
An enhancement which is included in underwater optical fiber cable having fibers tightly coupled to a core thereof is that of uniform recovery of the fibers along the cable length when loads imposed axially on the cable during manufacturing and installation are removed. For the long lengths of cable used in underseas applications, recovery in length following removal of installation tensile loads most likely will result in non-uniform length recovery of optical fibers which are not coupled sufficiently to other portions of the cable.
This problem may be solved by causing the optical fibers to be coupled tightly to a core member. In that way, the optical fibers and the core member move together and experience substantially the same strain. If this does not occur, then when loads are removed from the cable, the optical fiber may experience excessive undulations, resulting in microbending. Such a problem is exacerbated in underwater cables where the strains experienced are much greater than those experienced by terrestrial cables.
Seemingly, the prior art does not include an optical fiber cable which is well suited to an underwater environment and which includes a tight coupling between the optical fibers thereof and a core member as well as the capability for easy accessing of the optical fibers. The sought after cable should be one that may be manufactured inexpensively, yet one which is robust, and that may accommodate a plurality of optical fibers in a structure which results in low transmission loss. What is needed is a robust underwater cable which includes the tight coupling feature of the Mondello patent and the easy access features of slotted core cables.